I tend to consider all the casters to be pretty similar in movement overall, but that they represent different approaches to mobility. For example, BLM feels like a turret but a skilled player of the class knows that it has a ton of options you can use for on-demand movement. Similarly, RDM seems very mobile, but its mobility is more restricted in terms of when you can/must utilize it, and that tends to produce two very different playstyles. Then you have SMN, who is almost an aberration because you don't *technically* need to even hardcast anything at all in order to perform your DPS rotation (although playing that way will definitely result in a major DPS loss, it doesn't actually interfere with your class mechanics).

Personally, I enjoy RDM's playstyle the most.